1. Field of the Invention
The present invention relates generally to integrated circuit device packaging, and more particularly, to apparatuses and methods for improved bonding of solder balls to package structures.
2. Description of the Related Art
Typically, integrated circuit devices include multi-level structures contained within a substrate material. The multi-level structures can include passive components connected by metallization lines. In the field of radio frequency (RF) and wireless applications, use of Low Temperature Co-Fired Ceramic (LTCC) substrates are becoming more popular for defining multi-level structures having passive components connected by metallization lines. LTCC substrates are capable of embedding passive components while providing superior performance at high frequencies. In this manner, LTCC substrates are generally attached to a printed circuit board (PCB), a number of semiconductor devices, or a number of discrete components, or a combination thereof, to define a larger electronic device. The LTCC substrates are commonly attached to the PCB using a ball grid array (BGA) attachment technique. Attachment of the semiconductor devices to the LTCC substrate can be accomplished by flip chip or wire bonding.
FIG. 1A shows an illustration of a BGA-to-PCB attachment configuration 100, in accordance with the prior art. In the BGA-to-PCB attachment configuration 100, an LTCC substrate 101 is attached to a PCB 102 using a number of electrically conductive balls 111. Each of the balls 111 are disposed between a LTCC ball attachment pad 105 and a PCB ball attachment pad 106. Solder 108 is used to mechanically and electrically attach each of the balls 111 to both the LTCC ball attachment pad 105 and the PCB ball attachment pad 106.
FIG. 1B shows an illustration of a flip chip attachment configuration 120, in accordance with the prior art. A semiconductor device 121 is attached to the LTCC substrate 101 using a number of electrically conductive balls 127. Each of the balls 127 are disposed between an LTCC via (not shown) or an LTCC ball attachment pad 125 and a semiconductor under bump metallurgy (UBM) pad 123. The ball 127 can be pre-deposited onto the semiconductor UBM pad 123, the LTCC via, or the LTCC ball attachment pad 125. A solder reflow process is used to form joints between the semiconductor device 121, the LTCC substrate 101, and the balls 127. A fluxing agent is often used to aid joint formation during the solder reflow process.
In certain applications where device cost is secondary (e.g., military applications), gold (Au) conducting material is used within the LTCC substrate to fabricate the chip. However, in commercial applications where competition is a motivating factor for reducing cost, it is generally more desirable to use less expensive silver (Ag) conducting material within the LTCC substrate. Unfortunately, use of Ag conducting material within the LTCC substrate introduces material compatibility and component interface issues when using the BGA attachment technique. Specifically, use of Ag conducting material has traditionally required the use of a palladium (Pd)/Ag material mixture as the LTCC ball attachment pad 105. The Pd/Ag LTCC ball attachment pad 105 adhesion characteristics are adversely affected by reaction with solder materials during a typical device fabrication process. Consequently, the Pd/Ag LTCC ball attachment pad 105 is prone to delaminate from the LTCC substrate 101 resulting in BGA attachment failure during either fabrication or subsequent use of the device. Such BGA attachment failure causes product reliability to be unacceptably poor.
FIG. 1C shows an illustration of the BGA-to-PCB attachment configuration 100 with respect to the LTCC substrate 101, in accordance with the prior art. The LTCC ball attachment pad 105 in disposed above a via 103 in the LTCC substrate 101. The via 103 is a Ag or Ag/Pd conductor configured to electrically connect the Ag metallization lines (not shown) within the LTCC substrate 101 with the ball 111. In the prior art, the LTCC ball attachment pad 105 is composed of 20% Pd and 80% Ag. It has been traditionally assumed that the Pd enhances the resistance of the LTCC ball attachment pad 105 to leaching by the solder 108, wherein the solder 108 is composed of either a 96.5% tin (Sn) and 3.5% Ag mixture or a 63% Sn and 37% lead (Pb) mixture. It has been further assumed that the Pd inhibits Ag migration when exposed to a voltage bias. Also, the prior art suggests using a lower Pd content Pd/Ag mixture as a transition layer between the Ag via 103 and the Pd/Ag solder pad. The difficulty with using Pd/Ag for the LTCC ball attachment pad 105 as suggested by the prior art becomes apparent during fabrication when successive reflow operations are performed.
FIG. 1D shows an illustration of the BGA-to-PCB attachment configuration 100 with respect to the LTCC substrate 101 after a reflow operation, in accordance with the prior art. During the reflow operation, the Sn in the solder 108 diffuses toward the LTCC substrate 101. Correspondingly, the Ag/Pd material of the LTCC ball attachment pad 105 is displaced toward the ball 111. As a result of the Sn diffusion, a Sn diffusion layer forms within the LTCC ball attachment pad 105 and extends to the surface adjacent to the LTCC substrate 101. After successive reflow operations, the Sn diffusion layer can be composed of more than 50% Sn. Such a high percentage of Sn indicates a significant consumption of the LTCC ball attachment pad 105 through leaching by the solder 108. Thus, the presence of Pd in the LTCC ball attachment pad 105 does not provide enhanced resistance to leaching by the solder 108, as suggested by the prior art.
FIG. 1E shows an illustration of the Sn diffusion and resulting LTCC ball attachment pad 105 delamination 112 caused by the reflow operation, in accordance with the prior art. As previously discussed, the Sn contained within the solder 108 diffuses into the LTCC ball attachment pad 105 causing a displacement of the Pd/Ag toward the ball 111. The Sn diffusion layer formed within the LTCC ball attachment pad 105 at the LTCC substrate 101 interface weakens the adhesion between the LTCC ball attachment pad 105 and LTCC substrate 101. The weakened adhesion in combination with the mechanical and thermal stresses induced by the reflow operation causes delamination 112 of the LTCC ball attachment pad 105 from the LTCC substrate 101. Once delamination 112 occurs, the via 103 alone is required to withstand the mechanical and thermal stresses resulting from continued fabrication and subsequent use of the device. Generally, the via 103 is not strong enough to withstand these stresses. Thus, via 103 failure (i.e., cracking) causes the electrical conductivity from the via 103 through the ball 111 to be interrupted.
FIG. 1F-1 shows a scanning electron microscope (SEM) image of the BGA-to-PCB attachment configuration 100 following a typical reflow operation sequence, in accordance with the prior art. The LTCC substrate 101 is shown to be mechanically and electrically connected to the PCB 102 by a number of balls 111.
FIG. 1F-2 shows a SEM image of the ball 111 configured between the LTCC substrate 101 and the PCB 102 following a typical reflow operation sequence, in accordance with the prior art. The delamination 112 is visible on each side of the via 103 between the LTCC substrate 101 and the PCB 102.
FIG. 1F-3 shows a SEM image of the LTCC ball attachment pad 105 interface with the LTCC substrate 101 following a typical reflow operation sequence, in accordance with the prior art. The delamination 112 is clearly visible on each side of the via 103. Also, via failure 113 is visible at a location proximate to the LTCC ball attachment pad 105 interface with the LTCC substrate 101.
A prior art solution to the solder leaching, Sn diffusion, and solder pad delamination problems is to use gold (Au) or a Au containing mixture, such as Au/platinum (Pt) or Au/Ag, as the material for the LTCC ball attachment pad 105. One issue with this prior art solution is that the high cost of Au and Pt increases the overall cost of the device. Another problem is that use of a Au or Au containing material generally requires the use of a transition metal layer between the via 103 and the LTCC ball attachment pad 105. The transition metal layer is intended to limit the diffusion of Ag from the via 103 to the LTCC ball attachment pad 105. Such Ag diffusion can create voids in the via 103 resulting in an unacceptable loss of electrical conductivity. The transition metal layer is typically composed of Au/Ag or Pd/Ag. Both Au and Pd are expensive materials, thus the use of the transition metal layer increases the overall material cost of the device.
The transition metal layer can be implemented as either a buried capture cap or a post-fire cap over the via 103. Those skilled in the art generally consider the post-fire cap to be more reliable than the buried capture cap. Use of the post-fire cap, however, entails increased complexity in device fabrication. Specifically, use of the post-fire cap requires two additional firing operations. One additional firing operation is for the post-fire cap over the via 103. The other additional firing operation is for the Au or Au containing LTCC ball attachment pad 105 over the post-fire cap. Therefore, use of the Au containing LTCC ball attachment pad 105 and associated transition metal layer, as suggested by the prior art, results in an increased overall device cost due to higher material costs and increased fabrication complexity.
In view of the foregoing, there is a need for an apparatus and a method to reliably attach a BGA ball to an LTCC substrate. The apparatus and method should avoid the problems of the prior art by providing leach resistance, good adhesion, sufficient strength, simple fabrication, and overall cost effectiveness.
Broadly speaking, the present invention fills these needs by providing a ceramic package with solder ball attach pads that improve the performance of the ceramic package during manufacture and in-field use. The invention further provides a method for making a ceramic package having the improved solder ball attach pads. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several embodiments of the present invention are described below.
In one embodiment, a semiconductor ceramic package is disclosed. The package is defined by a ceramic body that has a plurality of conductive interconnect layers. The ceramic body has at least one solder ball attach side. A plurality of solder ball attach pads are defined on the solder ball attach side(s) of the ceramic body, and each of the solder ball attach pads is in contact with a conductive via that is in electrical communication with one of the plurality of conductive interconnect layers. Each solder ball attach pad includes metal content that is limited to silver metal. In this embodiment, a plurality of glass anchors are also provided. Each glass anchor is configured to surround and overlap a periphery of each of the solder ball attach pads.
In another embodiment, a package for semiconductor devices is disclosed. The package includes a low temperature co-fired ceramic body that has a plurality of conductive interconnect layers. The low temperature co-fired ceramic body includes at least one solder ball attach side. A plurality of solder ball attach pads are defined on the solder ball attach side(s) of the low temperature co-fired ceramic body, and each of the solder ball attach pads is in contact with a conductive via that is in electrical communication with at least one of the plurality of conductive interconnect layers. In this embodiment, each solder ball attach pad has metallic content that is limited to silver.
In another embodiment, a method for making a semiconductor package is disclosed. The method includes providing a ceramic body that has a plurality of metallic interconnect layers. The ceramic body has at least one solder ball attach side and a plurality of conductive vias. The method includes screen printing a solder ball attach pad over each of the conductive vias. The solder ball attach pad is defined from metallic content that is limited to silver. A glass anchor is then formed around an outer periphery of a number of the solder ball attach pads. The glass anchor overlaps at least a portion of the outer periphery and is partially defined over the ceramic body. In this embodiment, the glass anchor provides mechanical support to offset stress fractures at an interface between the solder ball attach pads and the ceramic body.
The advantages of the present invention are numerous. Most notably, the package of the present invention uses solder ball attach pads that limit the metallic content to silver. The solder ball attach pads are used to mechanically and electrically connect solder balls (e.g., BGA solder balls) to a ceramic body (e.g., an LTCC substrate). The use of silver as the sole metallic content of the solder ball attach pads minimizes solder leaching into the solder ball attachment pad during a reflow operation, wherein the leaching can cause a delamination of the solder ball attachment pad from the LTCC substrate. Accordingly, against convention, silver metallic material of the solder ball attachment pads provide for leach resistance, improved adhesion, improved mechanical strength, simplifies the fabrication process, and reduces cost for improved connection of the solder balls to the LTCC substrate.